Sensitization of photographic emulsions



United States Patent W SENSITIZATION or enolrodnarmc srous,

Charles F. H. Allen, Wesley G. Lowe, and John Sagal,

Jrl, Rochester, N.Y.,- assignors toEastman Kodak Com-' This invention relates to photographic silver halide emulsions, and more particularly, to a means for chemically sensitizing such photographic silver halide emul- 810115.

A number of methods have been previously proposed for increasing the sensitivity of photographic silver halide emulsions, other than methods of optical or spectral sensitization which involve the incorporation of certain dyes in the emulsions. The incorporation of dyes in the emulsions increases the optical range of sensitivity, whereas the incorporation or other materials known as chemical sensitizers increases the speed and generally the gamma of the emulsion without materially afiecting the optical range of sensitivity. Many of the prior methods of enhancing sensitivity, other than the methods of spectral sensitization, leave much to 'be desired because the increase of sensitivity may be quite small or the increase in sensitivity is accompanied by excessive fog and/or excessively poor keeping qualifies of the sensitized emulsions.

We have now found a method of increasing the sensitivity of photographic silverhalide emulsions wherein the speed increases are quite substantial, while the fog and keeping qualities are not excessively alfected, considering the magnitude of the increases in speed and gamma or contrast obtained.

It is, therefore, an objectof our invention to provide photographic silver halide emulsions which have been chemically sensitized. Another object is to provide a method of preparing such sensitized emulsions. another object of our invention is to provide photographic silver halide emulsions which have increased sensitivity, without concomitant increases in fog, or poor keeping qualities, to a degree which might seriously affect the usefulness of the sensitized emulsions. Other objects will become apparent from a consideration of the following description and examples.

According to our invention, we have found that the sensitivity of an ordinary photographic silver halide emulsion can be materially increased by incorporating a formamidine sulfinic acid therein. Unsubstituted formamidine sulfinic acid is sometimes known as thioureadioxide or imino-amino-methane sulfinic acid, depending upon the particular tautomeric form in which the compound is written. Formamidine sulfinic acid (or thiourea dioxide or imino-arnino-methane sulfinic acid) can be represented as existing in the following state of, equilibrium:

SOQH NHz -NH:(- NH2( 3 =NH The formamidine sulfinic acids used in our invention Still 2,983,610 Patented May 9, 1961 can be employed in the form of the free acid or in the form of their Water-soluble salts (e.g. sodium, potassium,

etc.). Formamidine sulfinic acids or salts which are particularly useful in practicing our invention can be:

represented by the following general formula.

R sonr wherein M represents a hydrogen atom or a solubilizing group or cation, such as sodium, potassium, etc., R, R and R each represents a hydrogen atom, a lower alkyl group (e.g., methyl, ethyl, n-propyl, n-butyl, etc., especially an alkyl group containing from 1 to 4 carbon atoms), an aryl group (e.g., phenyl, chlorophenyl, etc.,

especially a mononuclear aryl group of the benzene series), a heterocyclic radical (e.g., benzothiazolyl, piperidyl, etc.), etc.

The formamidine sulfinic acids useful in practicing our invention, or water-soluble salts of these acids, can be prepared in a known manner, such as the method referred to inWeber et al., US. Patent 2,339,309, issued January 18, 1944. i

It has been previously proposed to incorporate various sulfinic acids in photographic silver halide emulsions for the purpose of increasing the stability thereof. For ex ample, US. Patent 2,057,764 suggests the addition of alkane sulfinic acids, such as methane sulfinic acid, ethane sulfinic acid, etc., to photographic silver halide emulsions for the purpose of stabilization. However, as noted in this patent, these sulfinic acids have no detectable effect upon the sensitivity characteristics of the photographic silver halide emulsions. Contrasted with this behavior, the formamidine sulfinic acids of our invention have a profound efiect upon the sensitivity characteristics of the photographic silver halide emulsions.

The preparation of silver halide emulsions involves three separate operations: (1) the emulsification and.

digestion of silver halide, (2) the freeing of the emulsion of excess soluble salts usually by washing, (3) the second digestion or after-ripening to obtain increased sensitivity. (Mees, The Theory of the Photographic Process, 1942.) The sensitizers of our invention can be added to the emulsion before the final digestion or after-ripening, or they can be added immediately prior to coating.

The quantity of the particular formamidine sulfinic acid (or salt) added to the photographic silver halide emulsions can be varied, depending upon the effect desired, degree of ripening, silver halide content of the emulsion, etc. The amount used will also depend upon at what stage the sensitizer was added during the preparation of the emulsion. We have found that generally from about 0.02 to about 20 mg. of formamidine sulfinic acid per mole of silver halide is quite satisfactory to accomplish the sensitization desired, if added before the final digestion. The N-substituted derivatives can be employed in somewhat larger amounts, and generally as much as 250 to 300mg. can be added to the emulsions before digestion. On the other hand, somewhat larger amounts are required when the 'formamidine sulfinic acids or their salts are added to the emulsions immediately prior to coating. Under such conditions, from about 10 to 400 mg. of sensitizer can be 'added. The most useful oroptimum amount of sensitizer to be [used can bedetermined by preparing a series of test strips of the same emulsion,

wherein the quantity of sensitizer is varied 'Iti's to be halide emulsion is carried out for longer periods of time than normal, but at lower ripening temperatures. This observation has been particularly noticeable in the case of'formamidine sulfinic acid itself, andless noticeable in other cases.

There appears to be some reaction in the nature of a hydrolysis when the formamidine sulfinic acids of our invention are added to water, or aqueous alkaline solutions. Just what chemistry is involved when these sensitizers are contacted with water is not clear, since the aqueous composition resulting from such addition is a number of compounds difiicnlt to identify. Unsubstituted formamidine sulfinic acid appears to undergo spontaneous reaction with water, whereas certain of the N-substituted derivatives undergo reaction only when the water is warmed or alkali (e.g., sodium hydroxide, sodium carbonate) is added. Thus in certain instances, the sensitizers used in our invention can be characterized as hydrolysates of formamidine sulfinic acids.

The formamidine sulfinic acids useful in practicing our invention can be added to the emulsions to be sensitized either as a solid, or in admixture with water. When added as a solid, the formamidine sulfinic acid combines evidently with the water present in the emulsion to give the desired sensitizing effect. Alternatively, the particular formamidine sulfinic acid can be added to an aqueous alkaline solution, and either allowed to stand or heated slightly to effect solution.

The photographic silver halide emulsions used in practicing our invention are generally of the developing-out type. It is to be understood that the amount of silver halide in the emulsions can be varied, depending upon the particular purpose for which the emulsion is to be employed. Particularly useful results can be obtained when at least one formamidine sulfinic acid is employed in conjunction with gelatino-silver-bromiodide emulsions, although they can also be advantageously employed in sensitizing other silver halide emulsions, such as gelatinosilver-chloride, -bromide, -chlorobromide, -chlorobromiodide, etc., emulsions.

The photographic silver halide emulsions used in practicing our invention can also be optically sensitized by means of the dyes customarily employed in photography, such as cyanine dyes, carbocyanine dyes, dicarbocyanine dyes, merocyanine dyes, holopolar dyes, etc. The sensitizing effect obtained in our invention is not completely independent of the optical or spectral sensitizing dyes commonly used, although no adverse eflects have been noted in using emulsions which have been optically sensitized. Also, the formamidine sulfinic acids of our invention can be employed in conjunction with other chemical sensitizers.

The photographic silver halide emulsions employed in practicing our invention can also contain antifoggants or materials known to stabilize the emulsions against loss of speed under adverse keeping conditions, such as those customarily encountered in the tropics. Such antifoggants include, for example, those described in Allen et a1. U.S. application Serial No. 586,654, filed May 23, 1956, as well as many of the antifoggant compounds which can be generally classed as polyazaindenes. 1 Such polyazaindenes have been described in U.S. Patents 2,444,605-9; 2,743,181; 2,735,769; etc.

The photographic silver halide emulsions employed in our invention can be used in the preparation of materials, such as multilayer materials, intended for use in color emulsion layers are sensitized for a particular region of the spectrum. Such multilayer color materials generally have a red-sentitive layer coated on a support, followed by a green-sensitive layer, a blue light-absorbing layer, and a blue-sensitive layer, together with one or more interlayers comprising gelatin or some other colloid having various addenda, such as ultraviolet absorbing agents, etc., incorporated therein. These light-sensitive layers can contain color-forming materials of the type customarily employed in color photography, such as coupling compounds containing reactive ketomethylene groups, phenolic groups, or heterocyclic groups, such as pyrazolone. See, for example, U.S. Patent 2,640,776. These color-forming compounds can be incorporated in waterinsoluble, low molecular weight, organic, crystalloidal materials having a boiling'point above-about 175 C., such materials being of the type described in U.S. Patent It has been found that the chemical sensitizers of our invention are useful in combination with emulsions which have been sensitized with gold salts, such as those described in Waller and Dodd U.S. Patent 2,399,083, or Damschroder U.S. Patent 2,597,856 or Yutzy and Leerm'akers U.S. Patent 2,597,915 (e.g., potassium chloraurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride, Z-aurosulfobenzothiazole methochlon'de, etc.). Also, emulsions which have been sulfur- "sensitized, such as by the procedures shown in Sheppard U.S. Patent 1,574,944 or U.S. Patent 1,623,499 or Sheppard and Brigham U.S. Patent 2,410,689 can be employed in our invention. Such sulfur-sensitized emulsions can additionally contain gold sensitizers of the type men- "tioned above, or gold compounds intended primarily for Example 1 A fine-grain, low-gelatin, positive speed bromiodide emulsion was prepared and washed by the acid coagulation method described in U.S. Patent 2,614,928. The pH .of the emulsion was then adjusted to 6.0 and the emulsion held for 50 minutes at 60 C., both in the absence and presence of formamidine sulfinic acid. Additional gelatin and a coating aid, such as saponin, were then added and the emulsion coated on a suitable support, such as glass, cellulose acetate film, polyvinyl resin film, etc. After drying, the coatings were exposed for second positive on an Eastman Type Ib sensitometer and developed for 6 minutes at F. in a developer having the following composition:

. G. N-methyl-p-aminophenol sulfate 0.3 Hydroquinone 6.0 Sodium sulfite (dry) 38.0 Sodium bisulfite 1.5 Sodium carbonate monohydrate 22.0 Sodium bromide 0.78 Citric acid 0.7

Water to make one liter.

The speed contrast and fog for each of the emulsion coatings was measured, comparison being made between the coating containing no formamidine sulfinic acid, and

coating containing formamidine sulfinic acid in the quantities indicated below (the amount being on the basis of per mole of silver halide present). The results follow:

photography, where a number of separate silver halide ees w Example 2 A filiergfdi emul io P pare .i fixas yithe. mann shownin Example 1, except t atfthe tfinal digestion was for'l hour 'at 40 C., was treated in the presence and absence of formamidine sulfinic acid in the amounts indicated below. Speed, gamma and fog of each of the resulting coatings were measured as in Example -1 The Afine-grain photographic emulsion was prepared according to the method described in Example 2, except that the emulsion was then sulfursensitized,- for example, as described in U.S. Patent 1,574,944. A batch of this emulsion containing no formamidine sulfinieacid sensitizer was then compared with an identical batch containing the sensitizer in the amounts indicated below. Exposure and development was identical to that of Example 1. The results follow:

Formamldine Coating sulfinic acid 30/E ,Gamma Fog (mg/mole Speed (a) None 22. 80 3. 03 05 (b) 5. 52. 3. 80 .06

Examples 4, 5 and 6 below illustrate the efiects obtained when the formamidine sulfinic acids of our invention are added after the final digestion, but before coating.

Example 4 A high-speed bromiodide emulsion which had been chemicallly sensitized with sulphur and gold compounds was modified by adding thereto, after the final digestion but before coating, a formamidine sulfinic acid in the amount indicated in the followingltable. The emulsions were then coated on a conventional support, such as glass or cellulose acetate, and the coatings dried. The coated emulsions were then exposed in an Eastman Type Ib sensitometer for second and then processed for 5 minutes in a developer having the following composition:

N-methyl-p-aminophenol sulfate 2.5 Hydroquinone .f. 2.5 Sodium sulfite, (dry) 30.0 Sodium borate 10.0. Potassium bromide 0,5-

Water to make. one, liter.

The speed, contrast and fog,of each of the emplsion coatings was measured, comparison being made between the coating. containing no formamidine sulfinic acid and coatings containing a formamidine sulfinicI acid in the quantities indicated below (the amount, being, on the basis of per mole of silver halide present). The speed figure (CR) used in the table is; on the basis'of 100 (l -l og E), where E is the exposure in meter-candle: seconds of dayliglitquality radiationrequired to, produce a developed density. of 0.30gabove tog.

Fresh Tests Coating Addenda .(gJmole x x F. R Gamma 0 Speed None Formamidine sulflnle acid (.03)--.

N-methyl formamidlne sulfinie acid (.03).

N-methyl formamidlne sulflnlc acid (0.15),.

Example 5 In exactly the manner shown in Example 4 above, and

to the same type of emulsions, were added varying amountof ,foramidine su lfinic acids usefulin practicing our invention. Development was as described in Example 4. The following results were obtained:

Concen- Fresh Tests tration (g./mole AgX) Coating Addenda CR Speed Gamma Fog None Formamidine sulfinic acid Nzgzsethyl derivative of do .--..do

None (b) O. 03

Example 6 In exactly the manner illustrated in Example 4 above, an ordinary high-speed bromiodide emulsion was treated with onesof the compounds useful in practicing our invention. Instead of. adding the, compound directly to theemulsion as shown in Examples 4and 5, the tN-ethyl derivative shown in the table below was added to a dilute aqueous alkaline solution and warmed until solution was effected. The alkaline solution was then added to the emulsion immediately prior to coating and thoroughly mixed therewith. Processing was exactly as described in Examples 4 and 5 above. The results obtained were as follows:

Concentration (gJmole e Fresh Tests Coating Addenda GR Speed Gamma Fog N one 10 Alkaline hydrolysate of N ethyl formumidine sulfinio acid.

In Example 4. above, thepH of the emulsion was adjusted to about 6.3 prior to the addition of the formamidinesulfinic acid, while in Example 5,the pH was adjusted to about 7.1or 7.2 before adding thesensitizing compound In Example 6, the PH Of the emulsion used was about 6.3 prior to addition of the alkaline hydrolysate.

In Examples 7 and 8 above, the effect of several of the formamidine sulfinic acids useful in practicing our invention is illustrated.

Example 7 A fine-grain, low-gelatin, positive speed bromiodide emulsionpontaining no addedsulphur or gold sensitizers waslpr epared as described in Example 1 above. The emulsions were held at the. temperatures and times indicated in the following table, both in the absence and presence of a formamidine sulfinic acid. The digested coatings containing sensitizer were then coated on conventional cellulose acetate film and dried, The coatings were then exposed for & second on an Eastman Type 1b sensitometer and developed for 6 minutes at 70 F. in

H HHH l- HOIN HHH we:

a developer having the composition shown in Example 1 above. The speed (SO/E), gamma and fog for each of the coatings are given in the following table.

. s sions mayor may not contain couplers or color-formmg compounds, which combine with the oxidation proda nets of color, developers to produce colored images Coneen- V Coating Compound tration Finishing 30/11. Gamma Fog (mg/mole Conditions Speed AgX) None 2. 0. 99 04 Formamidine sulfinic acid 5.70 2. 50 .04 N-methyl derivative of formamidine sulflnie 3. 0 1. 68 04 ac a 1 Formamidine sulfinic acid 6. 90 2. 74 .04 N-miigahyl derivative of formamidine sulfinie 4. 75 2. 28 04 ac --do 6. 30 2. 72 .04

Example 8 A fine-grain, positive speed bromiodideemulsion was prepared and treated exactly as described in Example 7 above. The emulsions containing sensitizer were digested for the times indicated and their properties, after exposure and development, were compared with an untreated emulsion just as described in Example 7. The results obtained were as follows:

bearing a complementary relationship to'the exposing source.

As indicated above, the sensitizers of our invention can be employed in photographic silver halide emulsions containing stabilizers or antifoggants. For example, compounds comprising either the thiazolidine (I) or the tetrahydrothiazine (II) nucleus are useful antifoggants, contrast raisers, or sensitizers in photographic emulsions:

use of our sensitizers in photographic emulsions contemplates the use thereof in acid or alkaline emulsions; As indicated above, these photographic emulsions can be sensitized or not by means of optical sensitizing dyes. Such photographic emulsions can be orthochromatic, panchromatic or X-ray emulsions. If used with spectral (optical) sensitizing dyes, the sensitizers of our inven tion can be added to the emulsion before or after such dyes are added. Suitable dispersing agents for the silver halide emulsions according to our invention comprise gelatin or other colloids, such as collodion, albumen, cellulose organic derivatives, synthetic resins, etc. The sensitizers of our invention can also be used in emulsions adapted for use in color photography, such as emulsions coated in sequence on a single support and being sensitized to particular regions of the spectrum. The emul- Concen- Coating Compound tration Finishing 30/E Gamma Fog (mg/mole Conditions Speed AgX) (a) None None None 1.26 0.88 .04 (b) N-etiltilyl derivative of formamidine sulfinic 10.0 60 at 60 0.-.- 4. 73 2.72 .04

( do 20.0 -----do 5.40 2.70 .04

(d) do.-. 40.0 d0 6.60 3.16 .04

(e) do 100.0 60at40 3.75 2.70 .04

(r) d0 250.0 do 6.80 3.24 .04

The above examples illustrate the eflfect obtained with 5 1 our novel sensitizers in acid or alkalme emulsions. When employing these sensitizers in alkaline emulsions, 5 T T 1 5 E it is to be understood that alkali metal salts thereof may 4 CH 5 be'formed and that these salts exist in ionic form. As N indicated above, certain of these sensitizers appear to undergo reaction with the water, and for this reason they II I have been referred to above and in the following claims h nucleus i j i i h 3;4 i i to a fi e or as hydrolysates. Accordingly, 1t 1s to be understood that six-membered ring which may contain carbon, nitrogen,

oxygen, etc., e.g., in which:

Z /R0 (03911 CH-N R5 0:0 o=x i. n=1 01' Z ---R =H, alkyl, aryl, e.g. phenyl R and R =H, alkyl, phenyl, carboxyl, etc.

The photographic activity of atypical compound, the hydantoin of 4 carboxythiazolidine [n=1; R R

R H; X=0; Z=S inabove structure], is shown in the following table for two different emulsions.

Addendum Fresh Tests Two'Week Incubations 4 Concenat F./50% RH. Emulsion tration Per Mole of Silver OR Gamma Maximum Fog CR Gamma Fog Speed Density Speed 1 G. 2 Slow unsensitized Chloride g :fgg 83 23 8: 8i

0 290- 1.18 0.11 1.0 1 Negative type Bromoiodide 0,015 302 1.12 0.11 263. 0.94 0.46 0.030 291 1.02 0.09 251- 0.87- "0.50-

The hydrolysates used in our invention can be prepared as follows:

5.0 mg. of N-ethylthiourea dioxide were dissolved in cc. H O at room temperature, then 0.2 cc. of a .25 N sodium hydroxide solution were added and the solution heated to 60 C. as rapidly as possible on an electric hot plate and removed as soon as the above temperature had been obtained. The solution was then diluted to 20 cc. and used directly.

What we claim as our invention and desire secured by Letters Patent of the United States is:

1. An unexposed photographic silver halide emulsion sensitized by addition thereto of an aqueous hydrolysate of a compound selected from those represented by the following general formula:

wherein M represents a member selected from the class consisting of a hydrogen atom and an alkali metal atom, and R, R and R each represents a member selected from the class consisting of a hydrogen atom and a hydrocarbon radical, provided that at least one of the members selected from the group consisting of R, R and R represent a hydrocarbon radical, said aqueous hydrolysate having been prepared by heating a compound selected from those represented by said general formula in the presence of water.

2. An unexposed photographic silver halide emulsion sensitized by an aqueous hydrolysate of the N-methyl derivative of formamidine sulfinic acid, said aqueous hydrolysate having been prepared by heating the N- methyl derivative of formamidine sulfinic acid in the presence of water.

wherein M represents a member selected from the class consisting of a hydrogen atom and an alkali metal atom, and R, R and R each represents a member selected from the class consisting of a hydrogen atom and a hydrocarbon radical, provided that at least one of the members selected from the group consisting of R, R and R rep- -resents a hydrocarbon radical, said aqueous hydrolysate having been prepared by heating a compound selected from those represented by said general formula in the presence of water.

References Cited in the file of this patent UNITED STATES PATENTS 2,262,055 Sease et a1 Nov. 11, 1941 2,339,309 Weber et a1 Jan. 18, 1944 2,376,297 Weber May 15, 1945 

1. AN UNEXPOSED PHOTOGRAPHIC SILVER HALIDE EMULSION SENSITIZED BY ADDITION THERETO OF AN AQUEOUS HYDROLYSATE OF A COMPOUND SELECTED FROM THOSE REPRESENTED BY THE FOLLOWING GENERAL FORMULA: 